The primary goal of this proposal is to determine the mechanism by which CD43 regulates an inflammatory response in the lung. CD43 is a glycoprotein that is highly expressed on a variety of inflammatory cells including T cells, monocytes and neutrophils. We have demonstrated that ligation of CD43 augments the response of T cells to antigenic challenge. In addition, our study of CD43 deficient mice has shown that T cells from these mice are hyperresponsive both in vivo and in vitro. Furthermore, lung neutrophil recruitment is augmented in CD43 deficient mice following a single intra-tracheal challenge with LPS. These data point to a critical role for CD43 in the overall regulation of inflammatory processes. We hypothesize that unique structural features of CD43 allow it to function as a barrier to interactions between cell surface receptors and their ligands, either soluble or cell bound. We propose that a novel regulatory mechanism, the proteolytic cleavage of CD43, is a critical element in the function of this protein. To test this hypothesis and determine the precise mechanism of CD43 function, we propose the following specific aims: 1) Determine the role of proteolytic cleavage of CD43 in T cell activation and proliferation and 2) Determine the structural basis of CD43 function. In specific aim 1 we propose a definitive analysis of CD43 proteolysis in the activation of primary T cells. We have developed new reagents to allow for the detection of the detection of the multiple isoforms of CD43. Utilizing both normal and TCR transgenic mice we will determine whether CD43 ligation directly inducers its loss from the cell surface, and under what circumstances in normal T cell responses this occurs. In specific aim 2 we propose to undertake a detailed molecular analysis of the structural requirements that are essential for CD43 function. By the construction of mutant forms of CD43 and expression of them on a CD43 deficient background we will gain crucial information as to the mechanism of CD43's ability to modulate T cell activation specifically, and inflammation generally. The information obtained from these studies will further our overall understanding of the factors that control inflammatory responses that allow for the rational design of novel therapeutic strategies to treat a variety of inflammatory conditions.